This invention relates to a Doppler radar speed sensor for a land vehicle, comprising a radar transmitter/receiver circuit coupled to a transmit/receive aerial system for transmitting from said aerial system first and second beams of radio signals, derived from a common source, in first and second directions respectively both of which intersect the ground, and for processing reflected said radio signals received by said aerial system from said first and second directions to obtain information about the speed of said aerial system in a third direction which is parallel to the ground, said first and second directions making acute and obtuse angles respectively with a given sense of said third direction, said circuit comprising a mixer arrangement for mixing the received signals with a reference signal derived from said common source to produce a beat signal having a pair of Doppler frequency components the frequencies of which are representative of the speeds of said aerial system in said first and second directions respectively, and B digital data processing means including a spectrum analyser for determining the speed of the aerial system in said third direction from the spectrum of said beat signal.
One known sensor of this general kind is disclosed in EP-A-0371346. The aerial system of this known sensor is mounted beneath the vehicle in such manner that the first and second beams are directed towards the ground in first and second directions which nominally lie in a common plane. (In practice, of course, this ideal situation will not exist, due to the finite widths of B the beams). The plane is nominally vertical (perpendicular to the ground) and extends in the length direction of the vehicle, i.e. in the direction of forward/reverse travel (the "third direction"). The first and second directions lie on opposite sides of the nominal vertical and make equal angles therewith, a so-called "Janus" configuration. Thus one beam is directed forwards at an angle .theta. to the horizontal and the other beam is directed backwards also at an angle .theta. to the horizontal; one beam makes an angle .theta. with the forwards or backwards direction whereas the other beam makes an angle of 180.degree.-.theta. with the forwards or backwards direction. If at any given time the vehicle has a forward velocity v.sub.H and a downward velocity v.sub.V the Doppler frequency shift of the forwards beam will be 2f.sub.o (v.sub.H cos .theta.+v.sub.V sin.theta.)/c and the Doppler frequency shift of the backwards beam will be 2f.sub.o (v.sub.H cos (180.degree.-.theta.)+v.sub.V sin.theta.)/c=2f.sub.o (-v.sub.H cos .theta.+v.sub.V sin.theta.)/c, where f.sub.o is the frequency of the radio signals and c is the velocity of light. Thus the result of the mixing process is a beat signal having these Doppler frequency components. It will be evident that, provided f.sub.o and .theta. are known, v.sub.H can be calculated by determining the frequencies of these components and subtracting one from the other (thereby eliminating the effect of motion in a vertical direction. More particularly v.sub.H =c.DELTA.f.sub.D /(4f.sub.o cos .theta.), where .DELTA.f.sub.D is the difference between the two Doppler frequencies.
In the sensors described in EP-A-0371346 the mixer arrangement comprises a pair of mixers operating in quadrature, i.e. is a dual-phase arrangement. In the sensor shown in FIG. 5 thereof the output signals of the two mixers are applied, after analog-to-digital conversion, to real and imaginary signal component inputs respectively of a Fast Fourier Transform calculating arrangement which, as is known, constitutes in effect a bank of filters and in consequence effectively analyses the mixer output signals into their respective spectral components. The quadrature mixers are employed in order that, in known manner, a distinction can be made between positive and negative frequencies (Doppler frequency shifts of greater and less than zero respectively), the Doppler frequency shifts being effectively folded about zero in each mixer. The cost of such quadrature mixers and of pairs of components, e.g. filters, analog-to-digital converters etc. which may be required in the consequential pair of mixer output signal paths can, however, amount to an appreciable proportion of the cost of the complete sensor, and it is an object of the present invention to alleviate this problem.